Anode material for lithium secondary battery, an electrode for lithium secondary battery, a lithium secondary battery and the method of preparing anode material for lithium secondary battery

ABSTRACT

The present invention provides anode material consisting of anode active material having a great charge-discharge capacity, a high charge-discharge efficiency, a flat discharge curve and good charge-discharge cycle properties, and provides the anode material for a lithium secondary battery consisting of being coated with an amorphous metal compound formed by a metal capable of alloying with lithium on at least one part of the surface of a carbon material capable of absorbing and releasing lithium ion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a divisional of U.S. application Ser. No.09/670,888, which claims priority of application No. Hei 11-275380 filedin Japanese Industrial Property Office on Sep. 28, 1999, the content ofwhich is incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

[0002] (a) Field of the Invention

[0003] This invention relates to an anode and an electrode for a lithiumsecondary battery, a lithium secondary battery, and a method forpreparing an anode, and, more particularly, the present inventionrelates to the material of an anode, which is a metal compound coated ona carbon material.

[0004] (b) Description of the Related Art

[0005] With the proliferation of portable electronics devices in recenttimes, coupled with advancements which have enhances performance andenabled increasingly smaller sizes and weights for these devices,research is being actively pursued to improve the energy density ofsecondary batteries.

[0006] However, though graphite, one of the anode active materialstraditionally used in secondary batteries, has a theoretical capacity of372 mAh/g, in order to achieve higher capacity, multi-materials need tobe developed which are quite different from graphite or materials whichhave graphite as the main component.

[0007] For some time tin compounds have been examined for use as anodeactive materials in place of graphites. It is common knowledge that thetin of in these tin compounds can form an alloy with lithium and hasgreater capacity than graphite.

[0008] But, as compared with graphite, these tin compounds have lowinitial charge capacity, a non-plain discharge curve, and badcharge-discharge properties. Accordingly, tin compounds are not widelyused as anode active materials at the present time.

[0009] In spite of these defects, tin compounds are regarded as apromising anode active material because the tin compounds have muchgreater charge-discharge capacity than graphite.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide an anodematerial capable of serving as an anode active material having anexcellent charge-discharge cycle, a flat discharge curve, a highcharge-discharge efficiency, and a great charge-discharge capacity.

[0011] It is another object to provide a method for preparing an anodematerial, an electrode and a lithium secondary battery having the anodematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings wherein;

[0013]FIG. 1 is a cross-sectional drawing representing an example of alithium secondary battery of practical form according to this invention.

[0014]FIG. 2 is a drawing representing the result of X-ray diffractiontest of the anode material of Example 1.

[0015]FIG. 3 is a drawing representing the result of X-ray diffractiontest of the anode material of Example 2.

[0016]FIG. 4 is a graph representing the result of a 1 cycle chargedischarge test of a test cell using an anode material of Example 1.

[0017]FIG. 5 is a graph representing the result of a 1 cyclecharge-discharge test of a test cell using an anode material of Example2.

[0018]FIG. 6 is a graph representing the result of a 1 cyclecharge-discharge test of a test cell using an anode material ofComparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In the following detailed description, only the preferredembodiment of the invention has been shown and described, by way ofillustration of the best mode contemplated by the inventors of carryingout the invention. As will be realized, the invention is capable ofmodification in various obvious respects, all without departing from theinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature, and not restrictive.

[0020] In order to achieve the objects of the invention, this inventionconsists of the following.

[0021] The anode materials for a lithium secondary battery of thisinvention consist of coating amorphous metal compounds formed by metalsbeing capable of alloying with lithium on at least one part of a carbonmaterial, which is able to absorb and release lithium ions.

[0022] Particularly, it is preferable that anode materials for thelithium secondary battery of this invention are formed by coating a thinfilm of the metal compounds on at least one part of a carbon material.

[0023] The anode materials for the lithium secondary battery of thisinvention comprise metal compounds being capable of alloying withlithium, including one or more metal(s) selected from the groupconsisting of Sn, Fe, Pd, Pb, Al, Si, In, Ni, Cu, Co, Zn and Cd.

[0024] More specifically, the anode materials for the lithium secondarybattery of this invention may comprise metal compounds obtained byheating carboxylic metal salts.

[0025] It is preferred that the carboxylic metal salts are water-solublecarboxylic metal salts, for example, formic metal salt, acetic metalsalt, and propionic metal salt.

[0026] It is further preferred that the tin acetate is thermally stableand water-soluble among them.

[0027] It is more preferred that the carboxylic metal salt is(CH₃COO)₂Sn (tin acetate).

[0028] The metal compounds may be SnO, SnO₂, Ag₂O₂, AgCl, FeO, FeO₂,PbO, PdO, Al₂O₃, Al(OH)₃, SiO, SiO₂, InO₃, InCl₃, NiO, NiFe₂O₄, NiMoO₄,Ni(OH)₂, CuO, CuO₂, CuFe₂O₄, CuCl, CoO, CO₃O₄, ZnO, ZnAl₂O₄, CdO,CdSnO₃, etc.

[0029] Particularly, it is preferred that the metal compounds compriseeither SnO₂ or SnO, or both of them.

[0030] Moreover, the invention is characterized in that the electrodefor the lithium secondary battery of this invention includes one of theabove cite anode materials for the lithium secondary battery.

[0031] Although the final electrode consists of an anode formed bycombining the anode material, a conductor like graphite, and a binder inthe prescribed form, and then coating the anode material with acapacitor like a copper thin film, the current invention ischaracterized in that the lithium secondary battery of this inventionuses the above cited anode material for the lithium secondary battery.

[0032] The lithium secondary battery consists of a cathode, anelectrolyte, a separator, and an anode electrode (electrode) having theanode material according to the present invention. The lithium secondarybattery can come in various forms such as cylindrical type, coin type,sheet type, square type, etc.

[0033] The cathode electrode consists of a cathode active material, aconducting agent from carbon material, and a cathode material slurryformed by a binder.

[0034] For example, the cathode active material is a compound beingcapable of absorbing and releasing lithium, such as NiMn₂O₄, LiCoO₂,LiNiO₂, LiFeO₂, V₂O₅, TiS, MO, etc.

[0035] An olefinic porous film can be used as a separator, for example,such as polyethylene(PE), polypropylene(PP).

[0036] The electrolyte may be formed by mixing one or more lithiumsalt(s) selected from the group consisting of LiPF₆, LiBF₄, LiSbF₆,LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄,LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂) (C_(y)F_(2y+1)SO₂) (wherein x, y arenatural numbers), LiCl, Lil in one or more aprotic solvent(s) selectedfrom the group consisting of propylenecarbonate, ethylenecarbonate,butylenecarbonate, benzonitrile, acetonitrile, tetrahydrofuran,2-methyltetrahydrofuran, y-butyrolacton, dioxolane, 4-methyldioxolane,N,N-dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane,1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene,nitrobenzene, dimethylcarbonate, methylethercarbonate, diethylcarbonate,methylpropylcarbonate, methylisopropylcarbonate, ethylpropylcarbonate,dipropylcarbonate, diisopropylcarbonate, dibutylcarbonate,diethyleneglycol, dimethylether, may be used.

[0037] The invention is further characterized in that the method forproducing the anode material for the lithium secondary battery of thisinvention comprises the steps of: coating a carboxylic metal salt on thesurface of a carbon material by mixing carboxylic metal salt and carbonmaterial; transforming the carboxylic metal salt into an amorphous metalcompound by heating the carbon material having the carboxylic metalsalt, and forming a carbon material coated on one part by the metalcompound.

[0038] Particularly, it is preferred that the carboxylic metal salt isattached to the carbon material by coating the carboxylic metal saltsolution onto the carbon material and drying it, and then, heat-treatingthe carbon material having the carboxylic metal.

[0039] The invention also is characterized in that the carboxylic metalsalt comprises one or more metal(s) selected from the group consistingof Sn, Ag, Fe, Pd, Pb, AI, Si, In, Ni, Cu, Co, Zn, Cd, being capable ofalloying with lithium in the method for preparing of the above citedanode material of this invention.

[0040] Also, it is preferred that the carboxylic metal salt is awater-soluble carboxylic metal salt selected from the group consistingof formic metal salt, acetic metal salt, and propionic metal salt.

[0041] It is more preferred that the acetic metal salt is thermallystable and water-soluble.

[0042] One example is (C_(n)H₂₊₁COO)_(m)M, wherein, n is 0 to 2 ascomposite ratio, m is 1 to 4, and M is an element selected from thegroup consisting of Sn, Ag, Fe, Pd, Pb, AI, Si, In, Ni, Cu, Co, Zn, Cd.

[0043] Particularly, the preferable carboxylic metal salt is(CH₃COO)₂Sn.

[0044] In this example the invention is characterized in that the metalcompound is at least either SnO₂ or SnO, or both of them in the methodfor preparing of the above cited anode material for the lithiumsecondary battery.

[0045] Also, it is preferred that the heat-treatment temperature is 250°C. to 800° C. when using a carboxylic metal salt and tin in the methodof preparing of the anode material for the lithium secondary battery.

[0046] Also, the heat-treatment process is performed under non-activegas atmosphere or vacuum atmosphere.

[0047] Let it be explained that the lithium secondary battery ofpractical form of this invention with reference to the figure.

[0048] But, this invention is not restricted to the form represented inthe following figure.

[0049]FIG. 1 represents a lithium secondary battery 1, in one practicalform of this invention. The lithium secondary battery 1 of thisinvention is a cylindrical type, which consists of an anode electrode 2(electrode) of the sheet type, a cathode electrode 3 of the sheet type,a separator 4 arranged between the cathode electrode 3, and the anodeelectrode 2, an electrolyte impregnated in the separator 4, a batterycase 5 of the cylindrical type, and a sealing material sealing thebattery case 5.

[0050] The lithium secondary battery 1 further consists of accepting inthe battery case 5 a spiral type wound super positing anode electrode 2,a cathode electrode 3 and a separator 4.

[0051] The anode electrode 2 (electrode) related to this invention iscoated by a capacitor such that the anode material slurry comprising theanode material is coated by a copper film.

[0052] The anode material slurry comprises anode material, a conductorsuch as graphite, and a binder such as polyfluorovinyllidene for bindingthe anode material and the conductor.

[0053] The anode material of this invention is an amorphous metalcompound consisting of a metal capable of alloying lithium, which iscoated on at least one part of a carbon material capable of absorbingand releasing lithium ion.

[0054] Particularly, it is preferred that the thin film of metalcompound is coated on at least one part of the surface of the carbonmaterial.

[0055] The carbon material may be made of any material capable ofabsorbing and releasing lithium ion reversibly, or a mixture of one ormore of natural graphite powder, artificial graphite powder andamorphous carbon.

[0056] Also, it is preferred that the average diameter of the carbonmaterial is 6 μm to 40 μm, and it is more preferred that the diameter is8 μm to 25 μm.

[0057] The metal compound is one or more metal(s) selected from thegroup consisting of Sn, Ag, Fe, Pd, Pb, AI, Si, In, Cu, Co, Zn, Cd,capable of alloying with lithium. Particularly, it is preferred that themetal compound is a tin compound, and it is more preferred that themetal compound comprises either SnO₂ or SnO, or both of them.

[0058] An SnO₂ and/or SnO tin compound comprise(s) Sn easily capable offorming an alloy with lithium, and have(s) higher charge/dischargecapacity than carbon material.

[0059] The single use of this tin compound can form an anode materialhaving high charge/discharge capacity, but if this tin compound iscoated with carbon material, an anode material having the merits of boththe tin compound and the carbon material can be formed.

[0060] That is to say, on gaining a high charge/discharge capacity bythe use of tin compound, the excellent properties of highcharge/discharge efficiency, high cycle property and flat dischargecurve of the carbon material can be achieved.

[0061] This tin compound (metal compound) is formed by heat-treatment ofa carboxylic metal salt, particularly, it is preferred that the tincompound can be formed from a water-soluble carboxylic metal salt suchas formic metal salt, acetic metal salt, and propionic metal salt. It ismore preferred that the tin compound can be formed from (CH₃COO)₂Sn (tinacetate).

[0062] The amorphous metal compound can be formed by heat-treatment ofsuch a carboxylic metal salt.

[0063] In the case that the metal compound is (are) SnO and/or SnO₂, inregard to the weight ratio of the metal compound and the carbon materialin the anode material, the content of metal compound in the anodematerial is preferable less than 30 weight %, and more preferable 5weight % to 20 weight %.

[0064] It is not preferred that the content of metal compound (tincompound; SnO₂ and/or SnO) is more than 30 weight % because in such acase the charge-discharge property of the anode material is debased.

[0065] The cathode electrode is a cathode material slurry comprising acathode material coated on a capacitor such as an Al film.

[0066] The cathode material slurry is formed from a cathode material ofthe cathode active material, a conductor such as graphite, and a bindersuch as polyfluorovinyllidene, which binds the cathode material andconductor.

[0067] The cathode active material exemplifies a compound such asLiMn₂O₄, LiCoO₂, LiNiO₂, LiFeO₂, V₂O₅, TiS, MoS, capable of absorbingand releasing lithium ion.

[0068] Also, an olefinic porous film such as polyethylene orpolypropylene can be used as the separator 4.

[0069] A mixture solvating one or more electrolyte(s) of lithium salt(s)selected from the group consisting of LiPF₆, LiBF₄, LiSbF₆, LiClO₄,LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)C_(y)F_(2y+1)SO) (wherein, x, y are naturalnumbers), LiCl, LiF in one or more aprotic solvent(s) selected from thegroup consisting of propylenecarbonate, ethylenecarbonate,butylenecarbonate, benzonitrile, acetonitrile, tetrahydrofuran,2-methyltetrahydrofuran, y-butyrolactone, dioxolane, 4-methyldioxolane,N,N-dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane,1,2-dimethoxethane, sulfolane, dichloroethane, chlorobenzen,nitrobenzene, dimethylcarbonate, methylethylcarbonate, diethylcarbonate,methylpropylcarbonate, methylisopropylcarbonate, ethylbutylcarbonate,dipropylcarbonate, diisopropylcarbonate, dibutylcarbonate,diethyleneglycol, dimethylether, can be used.

[0070] Also, the solid type of polymeric electrolyte can be used inplace of the electrolyte solution, and it is preferred that a polymercan be used, and, for example, polyethyleneoxide, polypropyleneoxide,polyethyleneimine can be used. Also, the gel type of adding theelectrolyte and solvent on this polymer can be used.

[0071] Next, the method of forming the anode material of this inventionwill be explained.

[0072] The method of forming the anode material of this inventioncomprises the steps of attaching the carboxylic metal salt on the carbonmaterial by mixing the carboxylic metal salt with the carbon material,and forming one part of the carbon material coated with the metalcompound by transforming the carboxylic metal salt into the amorphousmetal compound.

[0073] Particularly, it is preferred that the carboxylic metal salt iscoated on the carbon material by drying the carbon material attachingthe carboxylic solution, and then heat-treating the carbon material.

[0074] The carboxylic metal salt used in this method is one or moremetal(s) selected from the group consisting of Sn, Ag, Fe, Pd, Pb, AI,Si, In, Ni, Cu, Co, Zn, Cd, capable of alloying with lithium.Particularly, it is preferred that the carboxylic metal salt is awater-soluble carboxylic metal salt such as formic metal salt, aceticmetal salt, and propionic metal salt. It is more preferred that thecarboxylic metal salt is water-solubility stable thermally among thecarboxylic metal salt.

[0075] In a preferred example the carboxylic metal salt represents aschemical formulae of (C_(n)H_(2n+1)COO)_(m)M. Where n of the chemicalformulae is 0 to 2, m is 1 to 4, and M is one or more element(s)selected from the group consisting of Sn, Ag, Pd, Pb, Al, Si, In, Ni,Cu, Co, Zn, Cd.

[0076] It is preferred that the carboxylic metal salt is tin acetate.

[0077] Also, it is preferred that the carbon material is capable ofreversibly absorbing and releasing lithium ion, such as, one or morematerial(s) of natural graphite powder, artificial graphite powder,amorphous carbon.

[0078] The method of precipitating the carboxylic metal salt on thesurface of the carbon material by inputting the carbon material in thecarboxylic metal salt and drying the solution is an example ofattachment-means of the carboxylic metal salt on the carbon material.

[0079] The method of precipitating the carboxylic metal salt on thesurface of the carbon material by spraying a solution of the carboxylicmetal salt on the carbon material and simultaneously drying the solutionis another example of attachment-means of the carboxylic metal salt onthe carbon material.

[0080] Particularly, the metal compound can be made into a thin-filmusing these methods, and the lithium for the carbon material can beabsorbed and released because it is easy for lithium ion to penetratethe metal compound in case of a charge-discharge reaction.

[0081] In the case where tin acetate is used as the carboxylic metalsalt, it is preferred that the weight % of tin acetate to carbonmaterial is less than 50 weight %, and it is more preferred that theweight % of tin acetate to carbon material is 10 weight % to 40 weight%.

[0082] If the content of tin acetate is more than 50 weight %, thecharge-discharge efficiency and cycle property are debased because thecontents of tin compound in the anode material is excessive.

[0083] Next, the metal compound is prepared by heat-treating carbonmaterial coated with carboxylic metal salt, and thermolyzing thecarboxylic metal salt. In case of using tin acetate as the carboxylicmetal salt, a tin compound comprising SnO₂ and SnO is formed as themetal compound.

[0084] In case of using tin acetate as the carboxylic metal salt, it ispreferred that heat-treatment temperature is 250° C. to 800° C., it ismore preferred that the temperature is 300° C. to 500° C.

[0085] If the heat-treatment temperature is less than 250° C., the tincompound of the perfectly thermolyzed tin acetate cannot be formed; andif the heat-treatment temperature is more than 800° C., the carbonmaterial can not comprise amorphous SnO₂ and SnO.

[0086] That is to say, the heat-treatment temperature in case of using acarboxylic metal salt, except for tin acetate, is determined for thatcarboxylic metal salt.

[0087] Also, it is preferred that the heat-treatment is performed undera nonreactive atmosphere or vacuum atmosphere.

[0088] By following these conditions, the anode material is formed if atin compound comprising amorphous SnO₂ and SnO is coated on the surfaceof the carbon material.

[0089] The anode material having all the merits of tin compound and thecarbon material is formed because the tin compound comprising amorphousSnO₂ and SnO is coated on the surface of the carbon material.

[0090] That is to say, the excellent properties of the lithium secondarybattery is gained because the lithium secondary battery simultaneouslyhas high charge-discharge efficiency, high cycle property and flatdischarge curve by the carbon material and high charge-dischargecapacity is gained by the use of the tin compound.

[0091] Also, the carboxylic metal salt can be attached on the surface ofthe carbon material and the anode material having uniform compositioncan be prepared because the water-soluble carboxylic metal salt is usedand the solution of the carboxylic metal salt is attached on the surfaceof the carbon material in the method of the anode material for lithiumsecondary battery.

EXAMPLE 1

[0092] A Preparation of Anode Material

[0093] Tin acetate solution was prepared by dissolving tin acetate 1 gin pure water 15 g. Next, this tin acetate solution was added to 10 g ofnatural graphite having an average diameter of 18 μm and then themixture was blended, and the tin acetate was attached on the surface ofthe natural graphite for 4 hours at 60° C.

[0094] And then, the tin acetate is thermolyzed by heat-treating thegraphite for 8 hours at 350° C. under a nitrogen atmosphere after saidprocess.

EXAMPLE 2

[0095] A Preparation of Anode Material

[0096] Tin acetate solution was prepared by dissolving tin acetate 50 gin pure water 300 g. And then, 250 g of natural graphite having anaverage diameter of 18 μm were inputted in an assembly container havingan impeller of a jetmill electric motion flow-layer assembly device, tinacetate was attached on the surface of the natural graphite by rotatingand agitating the natural graphite with 500 rpm of impeller insufflatingthe tin acetate solution in the natural graphite.

[0097] Next, after drying the graphite, the graphite was heat-treatedfor 8 hours at 400° C. under vacuum atmosphere and tin acetate wasthermolyzed. The anode material of Example 2 was prepared.

[0098] Anode Material of Comparative example 1

[0099] Natural graphite having an average diameter of 18 μm was used asthe anode of Comparative example 1.

[0100] Preparation of Test Cell for Charge-Discharge

[0101] Polyfluorovinyllidene was mixed with the anode material ofExample 1, Example 2, and Comparative example 1, and the slurry solutionwas prepared by adding N-methylpyrrollidone to the mixture.

[0102] This slurry solution was coated on a copper film of 18 μm by adoctor blade method, and N-methylpyrrollidone was volatilized by dryingit for 24 hours at 100° C. under vacuum atmosphere. By following thisprocedure, the anode material was laminated on a copper film of 120 μm.That is to say, the content of polyfluorovinyllidene of anode materialslurry was 10 weight %.

[0103] And then, the anode electrode was prepared by laminating theanode material and boring the copper in a circle with a diameter of 13mm.

[0104] The anode electrode was used as the active electrode, the lithiummetal film having a bore in a circle was used as the counter electrode,the separator formed from a porous polypropylene film between the activeelectrode and the counter electrode was inserted, and the test cell ofthe coin type using the electrolyte dissolving LiPF₆(1 mole/L) in amixture solution of propylenecarbonate(PC), diethylcarbonate(DEC), andethylenecarbonate(EC)(PC:DEC:EC=1:1:1) was prepared.

[0105] The charge-discharge test was then performed at 0.2 C ofcharge-discharge current density, 0 V(Li/Li⁺) of charge end voltage, and2.0 V(Li/Li⁺) of discharge end voltage.

[0106]FIG. 2 represents the result of X-ray diffraction of the anodematerial of Example 1, and FIG. 3 represents the result of X-raydiffraction of the anode material of Example 2. FIG. 4 represents theresult of a charge-discharge test of 1 cycle of a test cell using theanode material of Example 1, and FIG. 5 represents the result of acharge-discharge test of 1 cycle of a test cell using the anode materialof Example 2. FIG. 6 represents the result of a charge-discharge test of1 cycle of a test cell using the anode material of Comparativeexample 1. Also, table 1 represents a charge-discharge capacity and acharge-discharge efficiency of 1 cycle of each anode material.

[0107] The Test of Physical Property of Anode Material

[0108] Related to FIG. 2 and FIG. 3, the diffraction peak of SnO₂ andSnO, except for the diffraction peak of graphite in the anode materialof Example 1 and Example 2, can be confirmed. And, it can be confirmedthat the diffraction peak of SnO₂ and SnO is broad and SnO₂ and SnO areamorphous.

[0109] The energy peak of Sn was confirmed on performing the elementanalysis by X-ray analysis of an energy disperse type.

[0110] It is alleged that the anode material of Example 1 and 2 consistsof carbon material coated with amorphous SnO₂ and SnO.

[0111] The Result of Charge-Discharge Test

[0112] Also, as represented in table 1, it can be seen that the chargeand discharge capacity of the anode material of Example 1 and 2 ishigher than that of Comparative example 1. Particularly, it can be seenthat the discharge capacity of the anode of Example 2 is very highbecause the capacity is 426 mAh/g.

[0113] Also, it can be seen that the charge-discharge efficiency ofExample 1 and 2 is less than that of Comparative example 1, but both ofthem are not different. It is alleged that a drop of charge-dischargeefficiency is a constraint because the content of tin compound of theanode material is proper because the content of tin compound is under 50weight %. TABLE 1 Charge Discharge Charge- capacity capacity discharge(mAh/G) (mAh/g) efficiency (%) Example 1 487 404 83 Example 2 517 426 83Comparative 1 415 356 86

[0114] Also, as compared with FIG. 4 to FIG. 6, the discharge curve ofthe anode material (FIG. 6) of Comparative Example 1 represents thesteep change of voltage at the end of discharge, on the contrary, thedischarge curve of the anode material (FIG. 4, FIG. 5) of Example 1 and2 comparatively represent the gentle change of voltage at the end of thedischarge.

[0115] It is known that the voltage change is gentle when lithiumoccluded in SnO₂ and SnO reacts and is released. Thus, it is thoughtthat the release reaction of lithium occluded in SnO₂ and SnO can occurat the end of the discharge of the anode material of Example 1 and 2.

[0116] The leftover discharge capacity is recognized from thecomparative anterior step by sequentially detecting the change ofvoltage, because the voltage change of the anode material of the aboveExample 1 and 2 is comparatively gentle. Accordingly, the special effectthat graphite material can be used as the anode material, can beachieved.

[0117] As explained above, because the anode material for the lithiumsecondary battery of the invention is coated with an amorphous metalformed by metal capable of alloying with lithium metal on at least onepart of the surface of the carbon material, the lithium battery of theinvention can have both of the merits of the carbon material havingexcellent cycle property and flat discharge curve, and the merits of theamorphous metal compound having high charge-discharge capacity.

[0118] Also, the movement of lithium ion is not interrupted by the metalcompound because the carbon material is coated with a thin film of metalcompound and lithium ion penetrates the thin film of metal compoundduring the charge-discharge process, and the lithium ion for the carbonmaterial can be easily absorbed or released.

[0119] The charge-discharge capacity of the anode material can beenlarged because the metal compound comprises one or more metal(s)selected from the group consisting of Sn, Ag, Fe, Pd, Pb, Al, Si, In,Ni, Cu, Co, Zn, Cd, capable of alloying with lithium.

[0120] Moreover, the amorphous metal compound can be formed and thecharge-discharge capacity of the anode material can be enlarged becausethe metal compound is formed by heat-treating a carboxylic metal salt.

[0121] Also, it is possible for the charge-discharge capacity of theanode material to become high because the metal compound comprises SnO₂and/or SnO having very high charge-discharge capacity.

[0122] Moreover, because the lithium secondary battery of this inventionhas the lithium ion occluded in SnO₂ and SnO, at the end of thedischarge the anode material mainly performs a release-reaction and thevoltage change is comparatively gentle at the end of discharge,accordingly the leftover of discharge capacity is detected from thecomparative anterior step by detecting the voltage change sequentially.

[0123] Accordingly, the special effect that graphite material can beused as the anode material, can be achieved.

[0124] According to the method for preparing the anode material for thelithium secondary battery of the invention, because the method useswater-soluble carboxylic metal salt and the solution of carboxylic metalsalt is attached on the surface of the carbon material, carboxylic metalsalt can be uniformly attached on the surface of the carbon material andthe anode material having the uniform composition can be prepared.

[0125] While the present invention has been described in detail withreference to the preferred embodiments, those skilled in the art willappreciate that various modifications and substitutions can be madethereto without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

What is claimed is:
 1. A method for making an anode material for alithium secondary battery comprising: attaching a carboxylic metal salton a carbon material by combining the carboxylic metal salt with thecarbon material; and converting the carboxylic metal salt into anamorphous metal compound to form a carbon forming a metal compound onthe carbon material by transforming the carboxylic metal salt into anamorphous metal compound.
 2. The method according to claim 1, whereinthe step of attaching is performed by contacting the carboxylic metalsalt with the carbon material to form a coated carbon material anddrying the coated carbon material.
 3. The method according to claim 1,wherein the carboxylic metal salt comprises one or more metals selectedfrom the group consisting of Sn, Ag, Fe, Pd, Pb, Al, Si, In, Ni, Cu, Co,Zn, and Cd.
 4. The method according to claim 1, wherein the carboxylicmetal salt is tin acetate.
 5. The method according to claim 1, whereinthe metal compound comprises at least one of SnO₂ and SnO.
 6. The methodaccording to claim 1, wherein the carboxylic metal salt is convertedinto the amorphous metal compound by heat-treating the carboxylic metalsalt attached to the carbon material.
 7. The method according to claim6, wherein the heat-treating is performed at a temperature ranging fromabout 250° C. to about 800° C.